The invention relates to the formation of structures, and especially to the formation of sub-micrometer-scale or nanometer-scale structures by forming a layer of material and then concentrating that layer into a smaller area. The invention also relates to the self-assembly of sub-micrometer and nanometer length-scale substrate-based structures. The “structures” formed may be as simple as round, usually hemispherical or near hemispherical, micro- or nanoparticles on a substrate. They may also be highly faceted.
The rapid advances in solid state electronics over the last forty years are founded on the fabrication of wafer-based materials and interfaces with well-defined properties situated at well-defined locations where, over time, the density of structures has steadily increased as their dimensions have steadily decreased. This triumph of science and engineering is now being challenged by nanometer length-scales and quantum confinement effects. Establishing the ability to place arbitrarily small immobilized nanostructures on a substrate at predefined locations in a cost-effective manner is one of the grand challenges facing nanotechnology researchers. Addressing this unmet need would be transformative in that it would provide the enabling processing route required to advance new technologies based on electronic, optical and magnetic finite-size effects.
The two dominant techniques for forming patterned materials on sub-micrometer length-scales are photolithography and electron-beam lithography. These techniques are cost-prohibitive and face huge technical hurdles when creating patterns on length-scales less than 50 nm. An assortment of advanced lithographic techniques which are adaptations of these techniques are being implemented in an effort to overcome these limitations. Included among these techniques are 193 nm immersion lithography1, extreme ultraviolet lithography2, interference lithography and step and flash imprint lithography4. Associated with those techniques are technological and economic barriers which limit them to the most sophisticated fabrication facilities. One response to those technically demanding cost-prohibitive routes has been the development of a multitude of unconventional lithographic techniques focused on low temperature processing routes for the formation of patterned photoresist materials5-9. While far less cost-prohibitive, each of these routes faces its own distinctive technological hurdles. For all of the aforementioned techniques, defining nanostructures on sub-50 nm length-scales is proving difficult10.
In contrast, self-assembly processes have been quite effective in achieving the sizes required for the formation of substrate-supported nanostructures, but the accurate placement of self-assembled nanostructures has proved exceedingly difficult10-16.
Thin film and nanostructure formation on substrate surfaces is driven by thermodynamics and kinetics. While thermodynamics always drives the system towards the most energetically favorable state, atom kinetics often prevent this state from being reached. This is a simple consequence of the fact that atoms often encounter energy barriers that they have insufficient kinetic energy to overcome, a circumstance which confines the system to a metastable state. A continuous ultra-thin metal film deposited on a substrate at room temperature exists in such a metastable state if the surface energy of the metal is greater than that of the substrate material. Thermodynamics drives such a system towards a geometry which reduces the surface area of the metal. This geometry, however, is unattainable because the metal atoms lack the kinetic energy required to move significant distances across the surface. The net result is a continuous metastable film “frozen in place”.
Heating such a film allows it to move towards the equilibrium state, which sees it agglomerate into metal nanostructures at temperatures well below the melting point of the metal. This process, commonly referred to as dewetting, is typically initiated at grain boundary defects which give rise to holes that extend through the film to the substrate surface17. Agglomeration then proceeds through solid state surface diffusion away from these step edges. The process is quite complex with Rayleigh instabilities18, fingering instabilities19, grain boundaries20 and substrate surface texture18 all playing a decisive role. In general, thinner films dewet at lower temperatures forming nanostructures which are smaller and more densely packed21. The nanostructures formed are usually well-bonded to the substrate, having a contact angle determined by surface energy considerations (i.e. Young's equation22). The structure formed is also influenced by the crystallographic orientation of the substrate13 and film23, the lattice mismatch between the substrate and metal13, substrate surface reconstructions24, nanostructure faceting25 and interface chemistry26. When left at elevated temperatures for extended periods of time, the nanostructures are also subject to Ostwald ripening, which sees a disproportionate exchange of atoms between the nanostructures via surface diffusion that favors larger structures at the expense of smaller ones27,28.
The deposition of a high surface energy thin film on a low surface energy substrate, followed by its subsequent dewetting and agglomeration into droplets at elevated temperatures, has been used as a method for obtaining substrate-based nanostructures. This dewetting procedure typically occurs in the solid state at temperatures well below the melting point of the film material. The mechanisms governing this dewetting phenomenon, however, are quite complex and are influenced by the film thickness21, the crystallographic orientation of the substrate13 and film23, substrate surface reconstructions24, the substrate-film lattice mismatch13, faceting25 and interface chemistry26. This multitude of factors leads to limited control over and a randomness in the nanoparticle size distribution, spacing and placement.
One approach relies on the fabrication of a substrate with a periodic surface texture over which a continuous metal film is deposited17,29,30,63. At elevated temperatures the textured areas with the highest curvature create film weak points at well-defined locations that activate the dewetting phenomenon. While arrays of metal structures have been produced in this manner, the highly textured nature of the surface creates numerous challenges if these structures are to be incorporated into device architectures. Recently, we demonstrated that gold nanostructures can be sculpted from larger sub-micrometer structures produced via thermal dewetting31. The work demonstrated that capillary bridges formed between substrate immobilized gold structures and a metal foil resulted in a surface energy gradient able to drive gold diffusion towards the foil. Using this technique, periodic arrays of gold particles were reduced in size from 350 nm to a mere 21 nm. This technique, while successful, lacks the control required to sculpt nanostructures with predefined sizes.
The dewetting phenomenon is, by far, the simplest and most cost-effective approach for obtaining substrate-supported nanostructures. There are, however, a number of important disadvantages:
(i) A Substantial Size Distribution:
The quasi-random nature of the dewetting phenomenon results in a substantial nanoparticle size-distribution21 which negates the use of the resulting nanoparticles in applications where the collective response of an ensemble of identical nanoparticles is desired or where they act as catalytic seeds for obtaining identical nanowires via the vapor-liquid-solid growth mode32.
(ii) Lack of Control Over Nanostructure Size and Spacing:
As the film thickness is decreased the agglomerated particles become smaller21. This size reduction is accompanied by a disproportionate reduction in the average spacing between the nanostructures produced. This inability to tune the nanostructure spacing to the desired length-scale limits the ability to optimize plasmonic enhancements to photovoltaic33 and light-emitting devices34.
(iii) Dewetting Occurs Only for Select Film-Substrate Combinations:
The dewetting phenomenon is restricted to film-substrate combinations where the film surface energy is higher than the substrate surface energy. Even when that is the case, dewetting can be altered by the nature of the bonding between the film and substrate35.
(iv) Lack of Control Over Nanostructure Placement:
Once again, the quasi-random nature of the dewetting phenomenon provides little control over the final placement of the nanostructures produced, although the step-terrace structures offered by miscut substrates can impose partial order36.
Because solid state dewetting proceeds by surface diffusion away from edges that extend from the underlying substrate to the surface of the film, order can be induced into the agglomeration process through the establishment of lithographically patterned edges that, upon heating, retract in an organized manner21,37,38,60,63. Using this approach, in combination with epitaxial nickel films, Thompson and coworkers23,39,63 have induced the assembly of shaped micrometer-scale nickel islands where the final shape is dependent on (i) the annealing conditions, (ii) the epitaxial relationship formed and (iii) the orientation of the lithographically defined edges relative to the substrate crystal structure. This “passive” templating procedure, however, is limited by the fact that order is imposed by patterns having nanoscale features before the dewetting step. For example, when a film is deposited on a substrate and patterned to form a square with width ‘w’ and thickness ‘h’, there exists a pattern-width-to-film-thickness ratio (i.e. w/h) below which the film is able to agglomerate into a single structure when heated and above which the film breaks up into multiple structures. This ratio is, under ideal circumstances, often below 100. For gold structures on a sapphire substrate, the w/h ratio is typically limited to about 50. This ratio value is derived from the theoretical calculations of Dornel et al.60 utilizing the experimental gold-on-sapphire contact angle of 130°, but is also consistent with the present inventors' experimental findings. Calculations based on this value reveal that the final nanostructure cannot be smaller than about ¼ of the width of the original pattern. For example, pattern widths of less than 200 nm are typically required for the agglomeration of 50 nm nanostructures. Producing periodic templated structures on very small length-scales over large areas in a manner which preserves long range order becomes technically challenging and cost-prohibitive. As a result, conventional dewetting cannot economically produce the particle sizes that are desired for real-world applications.
The imposition of order onto the dewetting process through the use of lithographically-defined film edges23,63, periodic templates21,40 or substrate surface texture17,41,63 has been demonstrated with varying degrees of success.
An alternate approach uses easily fabricated micrometer-scale templates to produce an array of larger structures which are subsequently reduced in size through the use of surface energy gradients31 and/or high temperature anneals21. In principle, these templated dewetting techniques provide ways to fabricate large area nanostructured arrays. In practice, the areas defined by the template must become so small that template fabrication becomes the overriding obstacle. This impediment is further amplified by the disproportionate decrease in the areal extent over which agglomeration occurs as the film thickness is reduced21. Maintaining control over the process becomes increasingly challenging as the nanoparticle length-scale is reduced.
Conventional templated dewetting techniques can be used to fabricate arrays of micrometer-scale gold structures using the following procedure, illustrated by FIG. 1:    (a) A shadow mask consisting of an array of micrometer scale square openings is placed onto a sapphire substrate.    (b) A gold film is deposited onto the sapphire substrate through the shadow mask openings.    (c) The shadow mask is removed to leave well defined gold squares on the surface.    (d) The sample is heated to induce dewetting.
FIG. 2A shows an image of gold microstructures produced by the process of FIG. 1 when the film thickness is 160 nm. The shadow mask openings, and the resulting gold squares, are 5 μm across, positioned at 12.5 μm centers. Each square agglomerates into a single round particle approximately 2 μm across. This demonstrates how the simple procedure shown in FIG. 1 can be used to fabricate arrays through the conventional templated dewetting process. However, when the gold film thickness is reduced from 160 nm to 5 nm, instead of an array of single particles about 600 nm across, the same dewetting procedure instead yields as many as 40 smaller particles in each cell defined by the shadow mask. As shown in FIG. 2B, the particles are of uneven sizes and irregular spacing. That is a clear demonstration of the limitations of the conventional dewetting processes. A further thickness reduction to 0.5 nm produces a sample that shows nearly negligible agglomeration, as shown in FIG. 2C. Note that FIG. 2C is to a larger scale than FIGS. 2A and 2B, to show the very slight agglomeration more clearly. The scale bar is 20 μm in FIGS. 2A and 2B, and only 10 μm in FIG. 2C. This demonstrates that nanoscale particles are not readily fabricated through this procedure. The problem can in principle be resolved by reducing the size of the openings in the shadow mask to nanoscale dimensions, allowing a corresponding increase in the thickness of the gold film, but then the fabrication of the very fine mask becomes a major technological hurdle.